In recent years, portable electronic equipments, such as a portable telephone, a notebook computer, a personal digital assistant (PDA), a camcorder and a digital still camera, are largely spread, and with increasing demands of miniaturization and weight saving of the electronic equipments, a battery as a driving electric power source thereof receives increasing demands of miniaturization, weight saving, reduction in thickness and increase in capacity, for which studies are being actively made. A lithium secondary battery has a high voltage and a good energy density, and thus has been widely used as an electric power source of a portable electronic equipment. Associated with the development of the industry of displays having a small size and a light weight, however, there is a demand of a battery that has a smaller size and a lighter weight, and thus a lithium secondary battery is demanded to have enhanced battery characteristics including a higher driving voltage, a prolonged service lifetime and a higher energy density as compared to an ordinary lithium secondary battery. Middle-size or large-size lithium secondary batteries for automotive use, industrial use or the like are being developed in recent years, and there are expectations of developments for enhancing the capacity and the output power. For satisfying the demands, accordingly, there are being continuous efforts for enhancing the performances of the constitutional elements of the lithium battery.
The characteristics of the battery largely vary depending on the battery materials used, such as an electrode, an electrolyte and the like, and in particular, the characteristics of the electrode may be determined by an electrode active substance, a collector, and a binder, which imparts an adhesive force therebetween. For example, the amount and the species of the active substance used determine the amount of lithium ions capable of being bonded to the active substance, and thus a battery having a larger capacity may be obtained by using an active substance in a larger amount and by using an active substance having a larger inherent capacity. In the case where the binder has an excellent adhesive force between the active substances and between the active substance and the collector, electrons and lithium ions migrate smoothly within the electrode to reduce the internal resistance of the electrode, thereby performing charge and discharge with a high efficiency. A battery having a large capacity requires a composite electrode for an anode active substance, such as carbon and graphite, and carbon and silicon, which may suffer large volume expansion and contraction of the active substance on charge and discharge, and therefore the binder not only necessarily has an excellent adhesive force, but also necessarily has excellent elastic property and maintains the original adhesive force and restorative force even after undergoing considerable expansion and contraction of the electrode volume.
In view of the above points, a fluorine resin, such as polytetrafluoroethylene and polyvinylidene fluoride, dissolved in an organic solvent is used as a binder for providing an electrode. However, a fluorine resin may not have sufficiently high adhesiveness to a metal constituting a collector and may not have sufficiently high flexibility, and thus in production of a spiral wound battery, there are such problems that the resulting electrode layer suffers cracking, and the resulting electrode layer is released from the collector. Furthermore, the amount thereof used is necessarily large for maintaining the sufficient adhesive force, which may restrict miniaturization, and the use of the organic solvent mixed therewith disadvantageously makes the production process complicated.
A known binder that has high adhesiveness to a metal constituting the collector and is capable of forming a highly flexible electrode layer includes a binder formed of styrene-butadiene rubber (SBR) latex (see PTLs 1, 2 and 3). The binder is excellent in elastic characteristics but has a small adhesive force, and on repeated charge and discharge, the electrode fails to maintain the structure thereof, which may provide an insufficient service lifetime of the battery. In view of the demand of enhancement of the battery capacity in recent years, as for the materials constituting the electrode layer, the content of the binder component is decreased, and the electrode layer is press-molded in the production process of the electrode. In the electrode layer that has a small content of the binder component, however, the electrode layer is liable to be released from the collector in the press molding. The phenomenon not only causes contamination of the press molding machine with the electrode substance, but also provides such a problem that the reliability of the battery performance may be deteriorated when the electrode having an electrode layer that is partially released off is installed in the battery. The problem may be conspicuous when a polymer having a low glass transition temperature and high tackiness is used as the binder component, and thus the problem may be suppressed by using a latex formed of a polymer that has a high glass transition temperature, for example, higher than room temperature. However, the use of a binder formed of a polymer having a high glass transition temperature provides an electrode layer that has low flexibility and thus is liable to suffer cracking, which provides such a problem that the capacity retention of the battery is deteriorated to fail to provide sufficient charge and discharge cycle characteristics.